The social changes the Internet is bringing about have changed the way the two of us think about madness. The change in our thinking started, strangely enough, with reflections on Internet friends. The number of your Facebook friends, like the make of the car you drive, confers a certain status. It is not uncommon for someone to have virtual friends in the hundreds which seems to show, among other things, that the Internet is doing more for our social lives than wine coolers or the pill. In the days before Facebook and Twitter, time placed severe constraints on friendship. Even the traditional Christmas letter, now a fossil in the anthropological museum, couldn't be stamped and addressed 754 times by anybody with a full-time job. Technology has transcended time and made the Christmas letter viable again no matter how large one's social circle. Ironically, electronic social networking has made the Christmas letter otiose; your friends hardly need an account of the year's highlights when they can be fed a stream of reports on the day's events and your reflections on logical positivism or Lady Gaga.

It's hard to doubt that more friends are a good thing, friendship being among life's greatest boons. As Aristotle put it, "without friends no one would choose to live, though he had all other goods." But of course friends are only as good as they are genuine, and it is hard to know what to think about Facebook friends. This familiar idea was made vivid to us recently by a very depressed young woman who came to see one of us for the first time. Among the causes of her depression, she said, was that she had no friends. Sitting on her psychiatrist's couch, desperately alone, she talked; and while she talked, she Twittered. Perhaps she was simply telling her Twitter friends that she was in a psychiatrist's office; perhaps, she was telling them that she was talking to her psychiatrist about having no real friends; and perhaps â€” despite her protestations to the contrary â€” she was getting some of friendship's benefits by having a virtual community. In the face of this striking contrast between the real and the virtual, however, it's hard not to think that a Facebook or Twitter friend is not quite what Aristotle had in mind.

Still, one probably shouldn't make too much of this. Many of the recipients of the Christmas letter wouldn't have been counted as friends, in Aristotle's sense, either. There is a distinction to be made between one's friends, and one's social group, a much larger community, which might include the Christmas letter people, the colleagues one floor below, or the family you catch up with at Bar Mitzvahs and funerals. Indeed, the Internet is also creating a hybrid social group that includes real friends and the friends-of-friends who are little more than strangers. Beyond these, many of us are also interacting with genuine strangers in chat rooms, virtual spaces, and second lives.

In contrast with friendship, however, an expanded social group is unlikely to be an unalloyed good because it is hardly news that the people in our lives are the sources not only of our greatest joys but also our most profound suffering. The sadistic boss can blight an existence however full of affection from others, and the sustaining spouse can morph into That Cheating Bastard. A larger social group is thus a double-edged sword, creating more opportunities for human misery as well as satisfaction. A hybrid social group that includes near-strangers and true strangers may also open to the door to real danger.

This mixed blessings of social life seem to have been writ large in our evolutionary history. The last time social life expanded as significantly as it has in the last couple of years was before there were any humans. The transition from non-primates to primates came with an expansion of social groups, and many scientists now think that the primate brain evolved under the pressures of this novel form of social life. With a larger social group there are more opportunities for cooperation and mutual benefit, but there are also novel threats. Each member of a social group will get more food if they hunt together, for example, than they would get hunting by alone, but they also expose themselves to free riders who take without contributing. By living in larger social groups, the physical environment is more manageable, but deception and social exploitation emerge as new dangers. Since both cooperation and competition are cognitively demanding, those with bigger brains â€” and the concomitant brain power â€” will have the advantage in both. The evolution of human intelligence may thus been driven primarily by the kindness and the malice of others.

Some of the best evidence for this idea is that there is a relation in primates between brain size (more precisely, relative neocortical volume) and the size of the social group in which the members of the species live: bigger brain, bigger group. Plotting social group as a function of brain size in primates allows us to extrapolate to humans. The anthropologist, Robin Dunbar, calculated that the volume of the human cortex predicts a social group of 150 â€” about the size of the villages that would have constituted our social environment for a great deal of evolutionary time, and which can still be found in "primitive" societies.

How could one test this hypothesis? In non-human primates, membership in a social group is typically designated by mutual grooming. Outside of hairdressing colleges and teenage-girl-sleep-overs, this isn't a very useful criterion for humans. But the Christmas letter (or card) does better. Getting a Christmas card is a minimal indicator of membership in someone's social group. In an ingenious experiment, Dunbar asked subjects to keep a record of all the Christmas cards they sent. Depending on how one counted, the number of card recipients was somewhere between 125 and 154, just about the right number for our brains. It appears, then, that over the course of millions of years of human history our brains have been tuned to the social opportunities and threats presented by groups of 150 or so. The Internet has turned the human village into a megalopolis and has thus inaugurated what might be the biggest sea-change in human evolution since the primeval campfires.

We come at last to madness. Psychiatry has known for decades that the megalopolis â€” indeed a city of any size â€” breeds psychosis. In particular, schizophrenia, the paradigm of a purely biological mental illness, becomes more prevalent as city size increases, even when the city is hardly more than a village. And this is the case not because mental illness in general becomes more common in cities; nor is it true that people who are psychotic tend to drift toward cities or stay in them. In creating much larger social groups for ourselves, ranging from true friends to near-strangers, could we be laying the ground for a pathogenic virtual city in which psychosis will be on the rise? Or will Facebook and Twitter draw us closer to friends in Aristotle's sense who can act as psychic prophylaxis against the madness-making power of others? Whatever the effects of the Internet on our inner lives, it seems clear that in changing the structure of our outer lives â€” the lives intertwined with those of others â€” the Internet is likely to be a more potent shaper of our minds than we have begun to imagine.